A faction of South Sudan's armed opposition says it has replaced its leader who is also the country's first vice president, Riek Machar, amid uncertainty over his whereabouts.

Machar led a rebellion against President Salva Kiir in December 2013 but signed a peace deal last year, paving the way for him to return as Kiir's deputy.

But Machar fled the capital this month after Kiir's forces bombed his house during clashes that killed hundreds of people. Machar is now in hiding.

Machar's chief of staff, Ezekiel Lol Gatkuoth, said Taban Deng Gai, who had acted as the rebels' chief negotiator, would become first vice president until Machar returns.

Nyarji Roman, a spokesman for Machar's side who is also in hiding, said Saturday the move is a "conspiracy" to overthrow Machar.